Rock drilling is often carried out by percussion drilling, where a percussion piston, which is often operated hydraulically, is used to create a shock wave with the aid of an impact force that is generated by hydraulic pressure (percussion pressure), the shock wave being transmitted to the drill bit and hence to the rock through the drill steel (drill string). On contact with the rock, pins made of a hard alloy of the drill bit contacting the rock is pushed into the rock, generating a strong enough force to fragment the rock.
It applies in general, and especially in the case of drilling under difficult rock conditions and with a strong impact force, that the drill bit should have as good a contact with the rock as possible.
For this reason, the drilling machine is pressed against the rock: The drilling machine can be attached e.g. to a carriage, which moves along a supporting means, such as a feed beam, which is connected to a carrier, such as a vehicle. The drill bit is forced into the rock by moving the carriage, and therefore the drilling machine, along the feed beam towards the rock. The carriage can be operated e.g. by a hydraulic cylinder, which is usually called a feed cylinder. Alternatively, the drilling machine can be moved forward by using what is called a chain feed, in which case the feed cylinder is replaced by a hydraulic engine (feed engine), fitted with a spur gear. The carriage (the drilling machine) can then be moved forwards and backwards along the beam with the aid of a chain that is fixed to the carriage and is operated by the feed engine, where the chain runs along the feed beam. The hydraulic pressure that actuates the feed cylinder or feed engine is generally called the feed pressure.
The prevailing drilling conditions often change when a drilling machine of this kind is being used for drilling. There are many different types of rock, differing in drillability according to their quality, such as for example hardness. Soft and crumbling rocks are generally considered to represent the most difficult drilling conditions. The risk in the case of drilling into soft rock is that part of the energy of the shock waves is reflected when the rock is being hit and is transmitted back to the drilling machine along the drill string. The consequence of this is that the service life of the drill bit, the drill steel and the drilling machine can be reduced, with associated increases in cost as result. The drilling can be made even more difficult by conditions such as rock types of different hardness lying in a mixed arrangement in various beds.
It applies generally that an increase in the drilling speed (drilling rate) gives an indication that the rock is becoming softer. According to the prior art there are solutions utilizing this fact. In one of the known solutions, a throttle-type regulator or throttle is fitted at the return end of the hydraulic feed motor. If the drilling rate then becomes greater than is considered normal, the throttle begins to reduce the flow through the motor, so that a pressure difference is built up at the return end. The increase in pressure means that the pressure difference over the feed motor is reduced, and when a valve is used that is controlled by this pressure difference, and which in turn influences the percussion pressure, the percussion pressure can be reduced to an initial drilling level or essentially turned off completely when the drill bit enters a region of a softer rock.
Such a solution is described in EP 1,102,917 B1, with the difference that in this case the throttle is placed upstream of the feed engine. In this solution, the pressure over the throttle Valve is measured and is used to influence the percussion pressure if the pressure difference becomes too great.
However, the solutions described above have in common that they are difficult to set appropriately when the drilling rig is installed in a new drilling location. One reason for this is that, whereas a certain pressure difference over the feed motor or the throttle valve may be suitable for reducing the percussion pressure in one type of rock, a completely different pressure difference may be suitable in the case of another type of rock.
There is therefore a need for an improved method and device for controlling the percussion pressure, in particular when the drilling conditions change, which at least reduce problems encountered in the prior art.